<?php
/*
Plugin Name: Category not uncategorized
Description: Removes "Uncategorized" category from post if another category is assigned 
Author: Sam
Version: 0.1
*/

define('UNCATEGORIZED_CAT_ID', 1);
add_filter('category_save_pre', 'category_save_pre_remove_uncategorized');

function category_save_pre_remove_uncategorized($post_categories)
{
	if (count($post_categories) > 1) {
		$post_categories = array_diff($post_categories, array(UNCATEGORIZED_CAT_ID));
	}
	return $post_categories;
}

?>